[S-CG10_2AM2] Microcracks preceding ruptures in the crust related to earthquakes, volcanic eruptions and landslide

[SCG10-07] Induced seismicity due to fluid injection at a deep well in Youngstown, Ohio, USA

*金 元英¹ (1.Lamont-Doherty Earth Observatory, Columbia University)

キーワード：Induced seismicity, Fluid pore pressure, Shale gas extraction, Space and tme migration of earthquakes

Over 100 small earthquakes (Mw 0.4-3.9) were detected during January 2011- February 2012 in the Youngstown, Ohio, USA area, where there were no known earthquakes in the past. These shocks were apparently close to a deep fluid injection well, and hence, were immediately suspected as induced by the fluid injection. This 14-months seismicity included a half-dozen felt earthquakes and culminated with a Mw 3.9 shock on 31 December 2011, about 24 hours after the fluid injection ceased in the deep well in Youngstown. Among the 109 shocks, 12 events greater than Mw 1.8 were detected by regional network, whereas 97 small earthquakes (0.4 < Mw < 1.8) were only detected by using the waveform correlation detector. Among these shocks, 21 earthquakes were accurately located by using the local portable station data. All of the accurately located earthquakes were distributed along a set of subsurface faults striking N265 (due East-west) and dipping steeply to the north -- consistent with the focal mechanism of Mw 3.9 mainshock on 31 December 2011. All of the well-located earthquakes have occurred at depths ranging from 3.5 to 4.0 km in the Precambrian crystalline basement. We conclude that the recent earthquakes which occurred during 2011 - 2012 in Youngstown, Ohio were indeed induced by the waste fluid injection at a deep injection well due to increased pore pressure along the preexisting East-west trending faults located close to the wellbore in the Precambrian basement. We found that the earthquakes are located along a 1.2 km-long, East-west trending subsurface en echelon fault, and that the seismicity initiated at the eastern end of the subsurface fault -- close to the injection point, and migrated toward the west -- away from the wellbore, indicating that the expanding high fluid pressure front increased the pore pressure along its East-west trending path and progressively triggered the earthquakes. Further, we observe that the occurrence of these earthquakes is generally correlated to the total daily injection volume and that several sharp peaks in the daily injection volume correlate with the occurrence of earthquakes. We observed that several periods of quiescence of seismicity follow gaps in surface injection volumes and pressure (sudden drops in injection pressure followed by prolonged low pressure), which may indicate that the earthquakes were directly caused by the pressure buildup in the fractured Precambrian basement and stopped when pressure dropped. Geohydrologic properties of the Youngstown, Ohio area behaved as a fractured Precambrian rock similar to the Rocky Mountain Arsenal, Colorado, USA site of induced earthquakes during 1960s.